Experimental and Microstructural Study of Garnet Fracture Mechanics

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Whitney 0106673 A novel technique for determining the mechanisms, rates, and magnitude of decompression during unroofing of mountain belts, including quantifying the magnitude of high-temperature decompression and the unroofing pressure-temperature trajectory of formerly deep rocks, involves interpreting microcrack patterns in garnet. Of particular interest are microcracks that radiate from mineral inclusions, as their formation is likely initiated by decompression. By documenting the conditions at which the difference between lithostatic pressure and the internal pressure on mineral inclusions will fracture a host garnet in a radial pattern about the inclusion, it may be possible to calculate tectonic variables related to exhumation of mid- to lower-crustal rocks. This research involves (1) determination of the fracture toughness of silicate garnets as a function of temperature, composition, and crystal orientation, and (2) documentation and comparison of garnet microstructures in inclusion-rich and inclusion-free regions of garnets with well-constrained tectonic histories. Microstructures in the vicinity of inclusions surrounded by microcracks are compared with those near inclusions with no microcracks. Garnet properties will be determined using micromechanical testing apparatus, microstructures will be observed with high-resolution transmission electron microscopy, and structural domains will be assessed using electron backscatter diffraction techniques. Results from this research have applications to other decompressed materials (e.g., garnet-bearing mantle xenoliths) and provide fundamental information about the brittle behavior of garnet.